1. Field of the Invention
The present invention relates to a bonding apparatus and a bonding method having a process for judging whether a bonding state is good or bad, and in particular, relates to a bonding apparatus and a bonding method having a bonding state judging process, that performs, for example, wire bonding or die bonding to a semiconductor chip package etc. using ultrasonic vibration in fabrication of a semiconductor device.
2. Description of the Related Art
Conventionally, as a bonding method for a wire used in fabricating a semiconductor device, there have been known a nail head bonding type (thermal ultrasonic bonding type) using a gold wire, and a wedge bonding type using an aluminum wire.
The thermal ultrasonic bonding type using a gold wire that is strong against corrosion and excellent in expansibility has been frequently used for resin sealed semiconductor devices.
In a bonding apparatus of this type, a load to be applied to a semiconductor chip, a heating temperature, and the power and an applying time of ultrasonic waves to be used in combination with the heating can be set as parameters, and an ultrasonic oscillator in the bonding apparatus outputs the voltage corresponding to the set power for the set time to vibrate an ultrasonic horn.
As a result, a capillary attached to the tip of the ultrasonic horn is vibrated so that a gold wire is rubbed against an aluminum pad or inner lead of the semiconductor chip or the package so as to be bonded thereto.
However, even if the power and applying time of the ultrasonic waves are set, the degree of effectiveness of the ultrasonic waves changes depending on the condition such as the applied load, the mounting state of the capillary or the thermal expansion of the ultrasonic horn caused by the heating upon bonding, and thus does not precisely reflect the set values.
In view of this, various proposals have been presented to make it possible to monitor actual output voltage waveforms from a transducer upon bonding, thereby to monitor the bonding state.
FIG. 1 is a schematic diagram of a wire bonding apparatus disclosed in JP-A-H10-326811. An ultrasonic horn 23 is attached to a Z-movable mechanism 22 placed on an X-Y table 21, and a capillary 24 is retained at the tip of the ultrasonic horn 23. In the capillary 24, a gold wire (not shown) drawn out from a reel (not shown) is passed therethrough. To the other end of the ultrasonic horn 23 is coupled a transducer 25 that is connected to an ultrasonic oscillator 26, thereby to induce ultrasonic waves.
This vibration causes friction via the capillary 24 between the gold wire passing through the capillary 24 and a work 31, thereby realizing bonding therebetween. The foregoing operation is controlled by a bonder controller 29. Though not shown, there are further provided a torch for forming a ball at the tip of the gold wire and a clamper for retaining and tearing off the gold wire.
A voltage detector 27 is connected to output terminals of the transducer 25, and an arithmetic circuit 28 provided in the bonder controller 29 interprets a voltage waveform detected by the voltage detector 27. The arithmetic circuit 28 has a function of detecting a peak value P of the output voltage, a function of counting a voltage applying time T, a function of calculating the number n of waves equal to or greater than a transducer output voltage threshold value Vt set in the bonder controller 29, and a function of calculating the multiplication P·T·n as an ultrasonic effect index.
Herein, the voltage applying time T may be set as a time for which the voltage is, for example, 5% or greater than the expected maximum output voltage. This makes it possible to exclude a time period of extremely low output voltages so that the ultrasonic effect index well reflects an influence of the ultrasonic waves.
The value of the ultrasonic effect index P·T·n calculated by the arithmetic circuit 28 is displayed on a display unit 30 as a monitor via a display circuit provided in the bonder controller 29.
The first problem in the foregoing conventional bonding apparatus is that, although the output voltage of the transducer is detected by the voltage detector and the ultrasonic effect index is calculated by the arithmetic circuit based on the voltage waveform thereof and displayed on the display unit thereby to monitor the bonding state, a variation in mechanical vibration characteristic around the ultrasonic horn, i.e. deterioration of the capillary and so forth, can not be quantitatively seized from such an output voltage waveform.
The reason thereof is that inasmuch as the output waveform of the transducer is monitored, it is not possible to accurately seize a variation in mechanical vibration characteristic around the ultrasonic horn constituting the bonding apparatus.
The second problem is that, in the foregoing conventional bonding apparatus, although the output voltage of the transducer is detected by the voltage detector and the ultrasonic effect index is calculated by the arithmetic circuit based on the voltage waveform thereof and displayed on the display unit thereby to monitor the bonding state, since the output voltage of the transducer changes depending on a load applied to the work, the ultrasonic effect also changes when the applied load changes so that it is necessary to calculate again the reference ultrasonic effect index upon setting again the conditions of the wire bonding.
The reason thereof is that the ultrasonic effect index is the product P·T·n of the peak value P of the output voltage, the voltage applying time T and the number n of the specific waves, and does not rely on the applied load.
The third problem is that the time for replacing the capillary does not become definite.
The reason thereof is that when the calculation result of the ultrasonic effect falls outside the reference ultrasonic effect index, the bonding operation is stopped and only that defective chip is removed.
The fourth problem is that the ultrasonic effect index after the replacement of the capillary can not be calculated.
The reason thereof is that the ultrasonic effect index is calculated by the arithmetic circuit only upon actual bonding.
The fifth problem is that the ultrasonic effect index and a defective portion can not be seized.
The reason thereof is that the arithmetic circuit calculates the ultrasonic effect index and compares it only with the initial ultrasonic effect index.
The sixth problem is that it is necessary to calculate again the ultrasonic effect index upon changing the type of the works.
The reason thereof is that when changing the type of the works, it is necessary to calculate again the ultrasonic effect index upon initial bonding operation setting.